Give your dudes some move waypoints to make them wander around the camp (or wherever they are). Use a cycle waypoint to keep them going round in circles. After the cycle WP give them their get in and subsequent WPs. Now, the magic bit is to synchro your trigger with the cycle waypoint.
How to do that? Click on "Synchronise" at the top of the Mission Editor scree, click and hold on the trigger, drag the blue to the WP then let go the mouse button.
One more thing - make sure the trigger is type switch otherwise it won't work.
Bingo.
Edit: Aaarrgggh you beat me again SeAnVeR, I hate it when that happens. ;D
Anyway, either way will work